1. Field of the Invention
The present invention relates to a method of producing a pipe used for, for example, a heat exchanger or the like, by bending or the like, and also relates to an apparatus for implementing the same.
2. Description of Related Art
In recent years, space allocated to piping for the heat exchanger has a tendency to be reduced due to higher density within an engine room of an automobile and to increasing complexity of various types of heat exchangers of coolers or the like.
Additionally, under these circumstances, there has been a demand for minimizing the dead space for piping (space required for bending) and the bend R of the piping (i.e., radius of curvature of piping) to improve degree of freedom in piping layout.
However, when the bend R is made smaller in a bending process for piping, there occur problems that plate thickness of an outer-side portion of the bent pipe (hereinafter referred to as "a bend outer-side portion") may be excessively reduced and an inner-side portion of the bent pipe (hereinafter referred to as a "bend inner-side portion") may buckle.
That is to say, bending of a pipe is ordinarily performed using a pipe bender or the like, but a minimum radius of curvature is determined by a rate of elongation of pipe material, pipe outer diameter, pipe pad thickness, and the like, and there exists a problem that, when the pipe is bent with the minimum radius of curvature or less, the pipe is crushed, thus making impossible to perform bending.
To overcome such problem, an R-minimum elbow part can be obtained by joining two parts as shown in, for example, FIGS. 10A and 10B, however, there exist problems of an increase in work processes, a large increase in cost, and unsuitability for mass-production.
Additionally, separately from this, the technology disclosed in JP-A-61-137629 has been proposed for the purpose of reducing the cost and adopting in mass production. In this art, the pipe is bent while an elastic body is disposed within the pipe, and thereby making possible to perform bending of a small radius, however, there exists a problem that the troublesomeness of work is increased. That is to say, this method requires operations to insert and remove the elastic body, such as an operation for inserting the elastic body in advance in the pipe to be bent, an operation for maintaining the elastic body from a pipe end in opposition to the other pipe end to be machined in order to support the inserted elastic body, and an operation for removing the elastic body after machined, thereby causing problems that work efficiency may decline and cost may increase. Additionally, because it is necessary to maintain the elastic body, there also exist the problem that it is impossible to apply to parts with many bend points.
Furthermore, there have been proposed a method for performing spool machining to shape a curved portion after forming a pipe intermediate body provided with a curved portion as disclosed in JP-A-6-262282 and a method for performing bending by a workpiece which is shifted by sliding forward during bending in a clamp portion including a bending die and a clamping die as disclosed in JP-A-4-220120 or the like. Although each method may be effective with regard to preventing buckling of the bend inner side and enhancing circularity as one of problems for the minimum-R bending, it is impossible to prevent the cracking of the bend outer side as another problem. That is to say, because the formable minimum-bend R is determined by the intrinsic elongation rate of the material, it is possible to apply to materials with a large amount of elongation, such as stainless steel or mild steel, however, it is impossible to apply to material with small material elongation such as aluminum.